This invention relates to the design of nozzles employed in ink jet printing. More specifically, it relates to ink jet nozzles used for improved resolution and high resolution ink jet printers (printers having orifices on the order of 50 and 36 microns respectively). As is well known in this art, as the orifice size decreases the resolution increases, while the sensitivity of the printer to changes in the characteristics of the ink, operating temperature or frequency increases. This creates additional difficulties in the design of ink jet nozzles intended for high resolution printing
In a typical ink jet system, a nozzle is selected which has an acoustic resonance at approximately the operating frequency of the oscillator which is used to break a stream of ink into droplets. This operating frequency, referred to hereafter as "f.sub.0 ", is selected based on a number of operating parameters of the ink jet system including the desired resolution of the printer, the rate of dot matrix character formation, ink stream stability, etc.
Existing nozzles as, for example, the type disclosed in U.S. Pat. No. 4,727,379, assigned to the present assignee, and for which the present invention is an improvement, do not provide entirely satisfactory drop configurations for high resolution printing, particularly with certain inks. As is known in this art, satellites or small drops located between the main drops, can be generated when a stream of ink breaks up. Such satellites may degrade the quality of the printing process. These satellites can be forwardly merging, rearwardly merging or infinite. The first two terms indicate that during the flight of the ink drops, the satellites disappear prior to reaching the deflection field by merging forwardly with the main drops in front of them or rearwardly with the main drops that follow them. Infinite satellites do not merge at all and, depending upon the application, can interfere with proper printing.
Satellite problems are particularly acute for high resolution printers Such devices generally require a satellite-free ink stream. Rearwardly merging satellites however cause charge transfer between adjacent drops and are, therefore undesirable. Forwardly merging satellites produce a satellite free stream of drops entering the deflection field. Such condition permits precision placement of the drops on the substrate to be marked.
In standard, medium resolution, ink jet systems the nozzle is selected to have a single fluid resonance in its ink cavity which is closely matched to a desired nozzle operating frequency f.sub.0. This frequency matching permits operation of the nozzle using a relatively low stimulation voltage. On either side of the resonance are anti-resonant regions. The drive voltage necessary to operate the nozzle rises rapidly from the resonance point to values, at or substantially near the anti-resonances, which may exceed the capability of the transducer and its associated stimulation voltage source. Because of this relatively narrow operating frequency range, a typical ink jet system, when used for high resolution printing, is undesirably sensitive to changes in temperature, drive voltage or frequency drift.
It is accordingly an object of the present invention to provide an improved nozzle for high resolution ink jet printing which overcomes these disadvantages of the prior art nozzle designs.
It is also an object of the invention to provide a more robust operating region for low and standard resolution ink jet printers by using the principles of the invention.